Construction machines are called upon to perform a wide variety of tasks. A good example of such a machine is known as a backhoe and has a chassis which is often mounted on rubber-tired wheels, at least two of which are steerable. An operator's cab is supported by the chassis and the controls for the machine, e.g., the handles of hydraulic valves and the like, are mounted in such cab.
An articulated digging tool (which bears a resemblance to a human arm and hand) has one end of a boom mounted to the chassis for both "up-down" pivoting movement about a horizontal axis and rotating or "swing" movement about a vertical axis. The other end of the boom is hinge-connected to one end of a stick while a digging bucket is hinge-connected to the other end of the stick. The motion of the bucket with respect to its supporting stick is sometimes descriptively referred to as "curl."
In a backhoe, the bucket and its digging teeth face toward the chassis and the operator. Digging is achieved by urging the bucket teeth into the earth and moving the bucket toward the operator. When the bucket is filled, the operator "curls" it toward the stick and boom, raises it above ground level, swings the bucket to one side and, by curling in the opposite direction, empties its contents onto a pile or the like. As described below, digging and swing power are provided by hydraulic actuators. A hydraulic system for a backhoe-type excavator is disclosed in U.S. Pat. No. 4,838,756 (Johnson et al.).
Hydraulic actuators, e.g., rotary and linear motors (the latter usually called hydraulic cylinders) are separately controllable by the operator and separately power the swing movement, the up-down movement of the boom with respect to the chassis, the movement of the stick with respect to the boom and the movement of the digging bucket with respect to the stick. Motive power for the actuators is furnished by one or more hydraulic pumps drawing liquid, e.g., hydraulic oil, from a reservoir and delivering such liquid under pressure through a directional valve to a particular actuator or to particular actuators, in accordance with how the operator manipulates the controls.
If a backhoe is digging a trench in an open field, a high, maximum rate of swing is preferred for reasons related to machine "cycle time." The digging rate (and, therefore, productivity) are thereby improved.
On the other hand, if a backhoe is or is likely to be digging around or near a building foundation or wall or the like, it is desirable to limit the available swing rate to less than the maximum rate available for that particular machine configuration. In that way, the possibility of damaging the foundation or wall is greatly reduced.
The directional valve used by the operator to control swing rate is usually configured so that it can be "metered" or "feathered." That is, the rate of swing is a function of the position of the valve handle; moving the handle from its neutral to maximum offset position provides a continuum of swing rates from zero to the maximum available rate.
For an experienced machine operator, manipulation of the control handles and functions of a backhoe tend to be rather habitual, intuitive and "rhythmic." For that reason, neither the operator nor others prefer to rely upon the operator's skill and perception to, somewhat unusually, limit swing rate when working, e.g., near a building.
A known way to limit swing rate is run the engine and pump at wide open throttle and use an inlet restrictor, e.g., an orifice, between the pump and the inlet to the directional valve controlling the swing function. Under those operating conditions, the pump will deliver more hydraulic fluid than the orifice will accept. The remainder is "dumped" over a relief valve or the like. This approach results in a subtle but undesirable operating characteristic.
While the use of an inlet orifice in the foregoing manner will limit swing rate, it has no effect on the operating rates of the other functions, e.g., boom and stick extend or retract, bucket "curl," and the like. To state it another way, the maximum swing rate is, to an experienced operator accustomed to that machine, disproportionately low as compared to the maximum rates of the other functions. To the operator, the rhythm and intuition of operation are lost and productivity suffers.
An example will illuminate the foregoing. It is assumed that the hydraulic pump on a backhoe is capable of providing 25 gallons/minute (about 95 liters/minute) at a wide-open-throttle engine speed of 2300 rpm and of providing about 18 gallons/minute (about 68 liters/minute) at 1800 rpm engine speed. If swing rate is limited by reducing engine speed from 2300 rpm to 1800 rpm (which calculates to a reduction multiplier of 1800 divided by 2300 or about 0.72), the maximum rate of all of the other machine functions will also be reduced to 0.72 of their rates at higher engine speed. Rate "proportionality" is retained.
On the other hand, if engine speed is maintained at 2300 rpm and swing rate is reduced by using an inlet orifice as described above (and assuming the orifice will accept 18 gallons/minute maximum), the swing rate is reduced to 0.72 of its normal value. However, the rates of all of the other functions are maintained at their maximum rates at 2300 rpm engine speed. Function rate "proportionality" is lost.
The patent literature discloses a number of arrangements for controlling the operating speed of various functions in a construction or earthworking machine. For example, U.S. Pat. No. 4,838,756 (Johnson et al.) discloses an excavator hydraulic system having a pair of variable displacement pumps controlled by pilot operated load sensing control valves. There is a provision for placing one of the pumps in standby condition to reduce system flow capacity. U.S. Pat. No. 4,015,729 (Parquet et al.) discloses an automatic control system that controls pivot rate in a backhoe.
It is to be appreciated that another type of construction machine, known as an excavator, is closely similar in operation and configuration to a backhoe. A difference is that in an excavator, the bucket and its digging teeth face away from the chassis and the operator and digging is achieved by urging the bucket teeth into the earth in a direction away from the operator. But irrespective of this difference, control of swing rate control can also be important.
A hydraulic circuit and method which respond to the needs of the industry would be an important technological development.